LOS ANGELES NASA's Curiosity rover hasn't discovered any signs of methane in the atmosphere of Mars, a finding that does not bode well for the possibility that microbes capable of producing the gas could be living below the planet's surface, scientists said Thursday.

Since landing in Gale Crater last year, the car-size rover has gulped Mars air and scanned it with a tiny laser in search of methane. On Earth, most of the gas is a byproduct of life, spewed when animals digest or plants decay.

Curiosity lacks the tools to directly hunt for simple life, past or present. But scientists had high hopes that the rover would inhale methane after orbiting spacecraft and Earth-based telescopes detected plumes of the gas several years ago.

During Curiosity's first eight months on the red planet, it sniffed the air during the day and at night as the season changed from spring to summer.

"Every time we looked, we never saw it," said Christopher Webster, of NASA's Jet Propulsion Laboratory, who led the research published online in the journal Science.

Webster said while the result was "disappointing in many ways," the hunt for the elusive gas continues.

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NASA rover finds no methane on Mars in blow to search for life